This invention relates generally to photomask scanning and repair in semiconductor processing, and more particularly to the use of dual-focused ion beams for such scanning and repair.
Deposition and patterning are two of the basic steps performed in semiconductor processing. Patterning is also referred to as photolithography, masking, oxide or metal removal, and microlithography. Patterning enables the selective removal of material deposited on a semiconductor substrate, or wafer, as a result of deposition. The process of adding layers and removing selective parts of them, in conjunction with other processes, permits the fabrication of semiconductor devices.
This is shown by reference to FIGS. 1A-1D. In FIG. 1A, a layer 104 has been deposited on a semiconductor substrate 102. A layer of photoresist 106 is over the layer 104. A mask 108 is positioned over the photoresist 106, and has opaque regions 110 and 112. The base of the mask 108 is itself clear, and made out of glass. The opaque regions 110 and 112 of the mask 108 are formed out of chromium. Exposure involves the application of ultraviolet rays 114. The parts of the photoresist 106 that are not underneath the opaque regions 110 and 112 are exposed to the ultraviolet rays 114, and become polymerized as the photoresist 106xe2x80x2. The parts of the photoresist 106 underneath the regions 110 and 112 are not exposed to the rays 114, and remain unpolymerized.
In FIG. 1B, the polymerized photoresist 106xe2x80x2 is developed, which removes the photoresist 106xe2x80x2, leaving only the unpolymerized photoresist 106. The unpolymerized photoresist 106 has a pattern that corresponds to the opaque regions 110 and 112 of the mask 108 of FIG. 1A. In FIG. 1C, the layer 104 is etched to the substrate 102, such that the only part of the layer 104 that remains is that which is under the unpolymerized photoresist 106. This results in two stacks, a stack 116 and a stack 118. Finally, in FIG. 1D, the remaining photoresist 106 is stripped, leaving the stacks 116 and 118 of the layer 104 on the substrate 102.
The accuracy of the mask 108 is crucial for ensuring that the semiconductor devices formed are also accurate, and perform correctly. Defects in a photomask in particular can cause the semiconductor devices fabricated with the photomask to malfunction. Two common defects are shown in FIGS. 2A and 2B. In FIG. 2A, the mask 202 has a proper opaque region 204, but an improper opaque spot 206. Conversely, in FIG. 2B, the opaque region 210 of the mask 208 has an improper hole 212. Other common mask defects include inclusions of opacity into a clear region, protrusions of clarity into an opaque region, clear breaks within opaque regions, and opaque bridges between one opaque region and another opaque region.
Clear or missing parts of a mask are typically repaired by xe2x80x9cpatchingxe2x80x9d them with a carbon deposit. Opaque or unwanted chrome regions are usually removed by sputtering from a focused ion beam (FIB). One type of focused ion beam is a gallium ion beam. A focused gallium ion beam is capable of milling away opaque defects and depositing carbon film for clear defects at desired locations. The gallium ion beam may be used to help form the opaque regions on a clear mask, as well as to repair opaque and clear defects on the formed mask. The gallium ion beam is a positive ion beam, since gallium ions are themselves positive ions.
FIG. 3 shows a method 300 of the overall conventional approach that uses a gallium ion beam or other focused positive ion beam. First, the mask image is scanned using the positive ion beam to form the mask (302). This is also generally referred to as mask imaging or image scanning. Second, any defects in the mask are repaired, also with the positive ion beam (304). A difficulty with the conventional approach is that using a positive ion beam to perform mask scanning causes an excess of positive charge buildup on the mask, a phenomenon also referred to as the charge or charging effect. This positive charge buildup commonly reduces the effectiveness of the positive ion beam when performing mask scanning or repair.
One common problem is poor image quality, such as a faded or vague image, that results from the intensities of secondary ions and electrons being decreased as a result of the positive charge buildup. This is shown in FIG. 4A. The mask 402 has clear regions 404 and 406, and opaque regions 408 and 410. There should also be a clear spot 412 within the opaque region 408. However, it is not present, as indicated by the dotted-line nature of the spot 412, because the positive ion beam is not sufficiently efficient to neutralize the accumulated positive charge for isolated spots and patterns. This may require that a carbon film to be deposited to reduce the charging effect for the clear spot 412 to be properly formed.
Another common problem is that the charge buildup causes diversion of the positive ion beam during mask repair, which results in a loss of edge-placement accuracy because the ion bombardment position has shifted away from the desired location due to the diversion. This is shown in FIG. 4B. The mask 414 has a clear region 415 in which there are opaque regions 416 and 418. There should also be clear spots 420 in the region 416, and clear spots 422 and 424 in the region 418. However, because of the charge buildup, the clear spots 420, 422, and 424 have not been formed, as indicated by the dotted-line nature of the spots 420, 422, and 424.
To repair the mask, the ion beam is positioned over the desired locations of the spots 420, 422, and 424. However, the charge buildup diverts the beam. This causes the spot 420xe2x80x2 to be created within a newly formed opaque region 426, instead of the spot 420 to be created within the opaque region 416. Similarly, beam diversion causes the spots 422xe2x80x2 and 424xe2x80x2 to be created within newly formed opaque regions 428 and 430, respectively, instead of the spots 422 and 424 to be created within the opaque 418.
Therefore, there is a need for image scanning and mask repair that does not exhibit these problems. Specifically, there is a need for image scanning that does not result in charge buildup, and that does not result in vague or faded images. There is also a need for preventing ion beam diversion during mask repair. For these and other reasons, there is a need for the present invention.
The invention relates to the use of dual-focused ion beams for semiconductor image scanning and mask repair. A mask, such as a photomask, is imaged with either a focused negative ion beam, such as a focused oxygen ion beam, or a focused positive ion beam, such as a focused gallium ion beam. Mask imaging is also referred to as image scanning. Clear or opaque defects in the mask are repaired with the other ion beam that was not used in imaging of the mask. For instance, image scanning is performed with the focused negative ion beam to neutralize potential charge buildup, and mask repair is performed with the focused positive ion beam. The negative and position ion beams may be focused by an apparatus having a negative ion mechanism supplying negative ions, a positive ion mechanism supplying positive ions, a filter to select the desired ratio of the negative to the positive ions, and an aiming mechanism to focus the ions onto the mask.
The invention provides for advantages not found within the prior art. Imaging scanning does not result in positive charge buildup, or such buildup is neutralized, when a negative ion beam is used for mask scanning. This results in an image that is not faded or vague. Furthermore, any necessary mask repair can be performed by a positive ion beam without diversion of the beam, due to the lack of positive charge buildup. When a positive ion beam is used for mask scanning, a negative ion beam is used for mask repair, so that repair can be performed without diversion of the beam that otherwise results from using a positive ion beam for mask repair where there is charge buildup from also using the positive ion beam for the mask scanning. Still other advantages, embodiments, and aspects of the invention will become apparent by reading the detailed description that follows, and by referencing the attached drawings.